Like the Ghost says, in F3A precision aerobatics or "pattern" competition, the judges mark down your score if your speed is not constant during the manuver. Also if your loop is not perfectly round or if it exits at a different altitude than it starts at, or if it is not perfectly centered, or if your heading changes etc.

Also, while most trainers will stand the forces of constant full throttle, a lot of lightly built 3D planes will flutter if you use full power on a down line.